Recently, a storage device is combined with a power generator, for example, a solar cell, so as to be used as a power supply system. The power generator generates electric power using natural energy, such as sunlight, wind power, and hydraulic power. Such a power supply system formed by combining the storage device improves energy efficiencies by storing surplus electric power in the storage device and feeding electric power from the storage device when electric power is necessary in a load device.
One example of such a power supply system can be a photovoltaic system. In the photovoltaic system, the storage device is charged with surplus electric power when an amount of electric power generation by sunlight is more than an amount of electric power consumed by the load device. Conversely, when an amount of electric power generation is less than an amount of electric power consumed by the load device, electric power discharged from the storage device is fed to the load device in order to compensate for a shortfall of electric power.
In this manner, owing to the ability to store surplus electric power that has not been conventionally utilized in the storage device, the photovoltaic system is able to enhance energy efficiencies in comparison with a conventional power supply system.
Also, the charge and discharge control of the storage device is performed in such a manner that a remaining capacity (hereinafter, referred to as the SOC) indicating a state of charge of the storage device will not be increased to 100% in order to charge the storage device in the photovoltaic system efficiently with surplus electric power and the SOC will not drop to 0 (zero) in order to feed electric power to the load device whenever necessary. More specifically, the storage device is normally controlled in such a manner that the SOC varies in a range of 20 to 80%.
Such a principle is used also in a hybrid electric vehicle (hereinafter, abbreviated as HEV) using both the engine and the motor. In a case where an output from the engine is larger than motive power needed for driving, the HEV drives the electric generator with surplus electric power to charge the storage device. Meanwhile, the HEV charges the storage device by using the motor as the electric generator during braking or deceleration of the vehicle.
Recently, attention has been paid to a load leveling power supply and a plug-in HEV that effectively utilize nighttime electric power. The load leveling power supply is a system that consumes less electric power. It is a system that stores electric power in the storage device during nighttime hours when electricity charges are cheap and uses the stored electric power during daytime hours when electric power consumption reaches the peak. The purpose of this system is to maintain an amount of electric power generation constant by leveling an amount of electric power consumption, so that a contribution can be made to efficient operation of power equipment and a reduction of capital investment.
On the contrary, the plug-in HEV uses nighttime electric power. When the HEV runs in an urban area where fuel efficiency is poor, it is mainly driven by EV driving in which electric power is fed from the storage device whereas it is driven by HEV driving by which the engine and the motor are used when it runs over a long distance. The purpose of the plug-in HEV is to reduce a total amount of CO2 emission.
Incidentally, the storage device incorporated in the power supply system described above or the like is formed by connecting a plurality of storage elements (electric cells, unit batteries, etc.) in series. In the storage device formed in this manner, a capacity can vary from one storage element to another. In this case, when the storage device is discharged deeply at a large current, a storage element having a small capacity is over-discharged in comparison with other storage elements. Consequently, the overdischarged storage element deteriorates, which shortens the life of the storage device as a whole.
In order to suppress such deterioration of the life of the storage device, when a variance in capacity occur among the storage elements, the storage device is normally controlled so as to eliminate a variance in capacity using equalization means. However, when the storage device deteriorates, the capacity is reduced, which causes the internal resistance to rise. Accordingly, even when the capacities are made equal using the equalization means, a voltage drop becomes larger by the rising internal resistance when a large current is flown, and the voltage readily reaches the lower limit. Deterioration of the storage device is thus accelerated and the safety of the battery is degraded.
It is therefore crucial to detect deterioration of the storage device and the following methods are proposed as the detection method.
For example, Patent Document 1 discloses, as means for detecting deterioration of a battery, a method of discharging the battery by a predetermined amount after an equalization discharge process and determining deterioration of the battery on the basis of a voltage when the discharge ends.
Also, Patent Document 2 discloses, as a method of determining deterioration, a method of detecting voltages across blocks for a plurality of blocks (or cells) forming the storage device and determining an abnormality depending on whether a detected voltage difference exceeds a predetermined value.
Further, Patent Document 3 describes a determination method as follows. When the storage device deteriorates, an amount of discharge resulting from self-discharge increases while not in use (unused period), and so does an amount of voltage drop when the storage device is left unused over a long period. Accordingly, a voltage drop from immediately after the storage device is ended until it is started next (unused period) is calculated for each of the blocks forming the storage device and deterioration of a secondary battery is determined depending on whether a difference between the calculated voltage drop and the reference value exceeds a predetermined value.
The determination methods described as above, however, have the following inconveniences.
According to the method disclosed in Patent Document 1, because a predetermined amount of electric power is further discharged after the equalization discharge process, the state of charge of the storage device is deteriorated further. This raises a problem that an amount of energy (service life) that can be fed to the load device is reduced, which degrades the convenience of the device.
According to the method disclosed in Patent Document 2, a voltage difference at the time of detection is used for a determination. However, in a case where a variance in capacity occurs due to a decrease of the charge efficiency, a voltage difference caused by a variance in capacity is determined erroneously as being a voltage difference caused by deterioration. Accuracy of detection is therefore lowered.
According to the method disclosed in Patent Document 3, a determination is performed on the basis of an amount of voltage drop from the start to the end of the unused period. However, because a state (state of charge) at the start of the unused period varies from one block to another, a calculated amount of voltage drop is influenced considerably by the state of charge at the start. This makes it difficult to improve accuracy of abnormality detection.
Patent Document 1: JP-A-2003-282156
Patent Document 2: JP-A-11-178225
Patent Document 3: JP-A-2003-204627